In situ measurement of mass transport in (photo)electrochemical water splitting at device scale

Abstract

Understanding the transport of ions and dissolved gasses in aqueous solutions is crucial for the scaling-up of photoelectrochemical (PEC) energy conversion devices. In this study, we performed in situ measurements of the pH distribution and the dissolved oxygen concentration during water electrolysis, under realistic conditions for PEC water splitting devices. The influence of forced electrolyte convection on the mass transport processes was analyzed using particle image velocimetry (PIV) to capture the instantaneous electrolyte flow velocity fields. The measurements reveal a ∆pH of ~3 in 0.1 M KP_i buffer solution (pH 7) at 10 mA cm^-2, corresponding to a Nernstian overpotential of ~200 mV. While this detrimental loss can be mitigated by recirculating the mixed electrolyte, the use of higher buffer concentrations is found to be an even more effective strategy. At low flow rates, the pH gradient-induced overpotentials are significantly lower than those predicted by previously reported 1D multiphysics simulations. In situ observations of dissolved oxygen during water splitting demonstrate that the forced convection enhances oxygen removal and reduces the hazardous risk of product crossover. These quantitative findings can guide the rational design of highly-efficient, scalable PEC water splitting devices.